MOLECULAR CHARACTERIZATION OF MAJOR HISTOCOMPATIBILITY COMPLEX CLASS-II GENE-EXPRESSION AND DEMONSTRATION OF ANTIGEN-SPECIFIC T-CELL RESPONSE INDICATE A NEW PHENOTYPE IN CLASS II-DEFICIENT PATIENTS

Major histocompatibility complex (MHC) class II deficiency is an inherited autosomal recessive combined immunodeficiency. The disease is known as bare lymphocyte syndrome (BLS). BLS is characterized by a lack of constitutive MHC class II expression on macrophages and B cells as well as a lack of induced MHC class II expression on cells other than professional antigen-presenting cells (APCs) due to the absence of mRNA and protein of the human leukocyte antigen (HLA) class II molecules, designated HLA-DR, -DQ, and -DP. The defect in gene expression is located at the transcriptional level and affects all class II genes simultaneously. Here we have analyzed transcription and protein expression of class II antigens in Epstein-Barr virus (EBV)-transformed B lymphoblastoid cell lines and mononuclear cells (MNCs) of twin brothers. Whereas flow cytometric analysis failed to detect class II antigens on the cell surface of the patients' EBV-B cells and MNCs, examination of the genes coding for HLA-DR, -DQ, -DP, and the invariant chain (Ii) by reverse transcriptase-polymerase chain reaction amplification resulted in an unusual mRNA pattern in the B cell lines of the patients (HLA-DR alpha(+), -DR beta(-), -DQ alpha(+), -DQ beta(-), -DP alpha(-), -DP beta(+), Ii(+)). In accordance with these findings no HLA-DR beta-specific protein was detected by immunoblotting, whereas low levels of HLA-DR alpha and normal levels of Ii were present. In contrast to EBV-B cells, the MNCs of both patients displayed a residual HLA-DR beta, -DQ beta, and -DP alpha mRNA signal. Furthermore, HLA-DR beta-specific protein was found in addition to HLA-DR alpha by immunoblotting of cell lysates, even though it was clearly decreased as compared with controls. Our results indicate that the defect in class II antigen expression is not necessarily present to the same extent in B cells and cells of other lineages. mRNA levels of HLA-DR beta were found to be enriched in adherent cells within the MNC fraction. Further investigations indicated that the MHC class II expressed is functional in antigen presentation, as the two boys' CD4(+) T cells became activated and expressed interleukin-2R after stimulation of peripheral blood mononuclear cell cultures with recall antigen (tetanus toroid). Furthermore, T cells tested in one of the two patients responded to both MHC class I and II allostimulation, and this response was inhibited by monoclonal antibodies of the respective specificity. Whereas the MNC population contained sufficient APCs to activate CD4(+) T cells in response to antigenic stimulation, the patients' EBV-B cells were unable to present recall antigen to autologous, long-term cultured, antigen-reactive T cells or to a normal, HLA-DR-compatible, antigen-specific T cell line. In contrast, the patients' EBV-B cells functioned normally as accessory cells for mitogen-induced T cell proliferation. The results obtained from the investigations of MHC class II-dependent immune functions indicate antigen presentation by a subset of cells, obviously present in the HLA-DR beta mRNA-expressing adherent MNC population, whereas the patients' EBV-B cells lack this ability.